Electroacoustic transducer



I March 5, 1968 J cYR' 3,372,370

7 ELECTROACOUSTIC TRANSDUCER Filed Sept. 22, 1965 INVENTOR. REG/MALL) J.CYR

THOMAS P MAHONEY A 77'0E/VEY United States This invention relates to anelectroacoustic transducer of the piezoelectric type adapted to beutilized in deep underwater applications where extremely high pressuresare encountered of an order so great as to prevent the accuratefunctioning of conventional prior art electroacoustic piezoelectrictransducers.

' It is well known to those skilled in the art that a piezoelectrictransducer of any kind must be so mounted in association with itsaccompanying housing that it will perform throughout its frequency rangewithout being damped in such a way by the rigidity of the mounting orthe impression thereupon of ambient conditions as would prevent theproper functioning throughout its frequency range.

It has previously been known that piezoelectric acoustical transducers,when mounted in conventional packaging, have not functioned adequatelyat extreme subaqueous depths because the packaging of the transducershas been such as to permit the impression thereupon of the force createdby the extreme pressures to which the transducers have been subjected,thus preventing them from functioning properly throughout theirfrequency range and militating against their use in deep underseaapplications.

For instance, prior art piezoelectric transducers have made use ofvarious types of foam material, such as polyurethane and foam rubber andthe compressibility of such mounting materials, while satisfactory forordinary applications, has not been sufiicient to prevent the impositionof excessive forces generated by the extremely high pressures upon thepiezoelectric transducing elements incorporated in said transducers.

It is, therefore, an object of my invention to provide a piezoelectrictransducer peculiarly adapted for use in deep undersea applicationswhich is packaged in conjunction with a pressure-accommodating masswhich has been found particularly adaptable for sustaining highhydrostatic loading and, at the same time, serving as an acoustic energyabsorbing material to protect certain surfaces of the piezoelectrictransducing element, thus permitting the piezoelectric transducingelement to function throughout its design range and to familitate itsuse in deep undersea applications.

Another object of my invention is the provision of an electroacoustictransducer of the aforementioned character which includes apiezoelectric transducing element which has operatively associatedtherewith a pressureaccommodating mass which includes a supporting body,

matrix or lattice of organic plastic material, such as epoxy-resin, andwhich incorporates a high percentage of nonorganic hollow spheroids, thecombination of said piezoelectric transducing element andpressure-accomodating mass being encompassed in an appropriate housing.

An important object of the invention is the provision of apressure-accommodating mass which is an acoustical energy absorber andis sufiiciently compressible to allow response to a weak alternatingsound field while still providing the required pressure release oraccommodation internally of a cylindrical transducer.

An additional object of my invention is the provision of anelectroacoustic transducer of the aforementioned type in which thehollow spheroids are fabricated from glass, ceramic, or similarnonorganic materials and are of dimensions of several microns indiameter. These atent spheroids are known to the art, when manufacturedout of such materials as polyurethane and the like, as microballoons,and will hereinafter be referred to by that term.

Other objects and advantages of the invention will be apparent from thefollowing specification and the accompanying drawing, which is for thepurpose of illustration only, and in which:

FIG. 1 is an isometric view illustrating a transducer manufactured inaccordance with the teachings of this invention in its simplest form:

FIG. 2 is a vertical, sectional view taken on the broken line 2-2 ofFIG. 1;

FIG. 3 is an enlarged, fragmentary, sectional view showing themechanical arrangement of the constituents of the pressure accommodatingmass; and

FIG. 4 is an enlarged, fragmentary, sectional view showing the manner inwhich the organic, synthetic plastic lattice or body of thepressure-accommodating mass encompasses the microballoons and serves tosupport them in operative relationship with one another.

Referring to the drawing, and particularly to FIGS. 1-2 thereof, I showan electroacoustic transducer 10 constructed in accordance with theprinciples of my invention and including a housing 12 which may beplaced about the piezoelectric transducing element 14 and may beselected from any one of a large variety of suitable encapsulatingmaterials, such as polyurethane, neoprene, and the like.

The piezoelectric transducing element 14 is illustrated as beingconstituted by a substantially cylindrical body 15. For instance, a leadzirconate-lead titanate ceramic cylinder has been utilized whoseover-all diameter is one inch, whose height is one inch, and whose wallthickness is .125 inch. The ceramic body 15 incorporates a centrallylocated bore 16, and the bore has a pressure-accommodating mass 18deposited therein, in a manner to be described in greater detail below.

While the pressure-accommodating mass 18 has been disclosed as depositedin the bore 16 of the piezoelectric element 14, it is also possible andfeasible to completely encompass the piezoelectric element 14 in thepressureaccommod-ating mass. This is particularly the case when apiezoelectric element having a configuration, such as a diskconfiguration, is utilized in substitution for the cylindrical type ofelement 14 disclosed herein. The pressure-accommodating mass 18 willperform two functions. It will serve as an encapsulating as well as apressure release medium.

In addition, the mass 18 can be applied directly to the exterior orvibrating surface of a ceramic transducer of a given geometric shape andthickness to produce a shaded operational pattern. This pattern will befixed regardless of depth.

Moreover, the pressure-accommodating mass may be used in conjunctionwith line transducers as a gasket material between the ceramiccomponents thereof to decouple said components from one another.

The pressure-accommodating mass 18 includes a supporting body 20 whichmay be fabricated from an epoxyresin or a similar synthetic organicplastic material and has deposited therein a plurality of microballoons22 which are hollow spheroids of a few microns in diameter, which arefabricated from glass, ceramic, or other nonorganic materials. Themicroballoons 22 are disposed in close juxtaposition to one another, andare supported by a relatively thin lattice work constituted by thesupporting body 20 of plastic material.

The pressure-accommodating mass may be fabricated by utilizing parts ofepoxy-resin, 10 parts of an appropriate catalyst, and a high percentageof microballoons mixed in the fluid resin before it has catalyzed. Byweight,

3 I have found that 4 to 8 grams of microballoons mixed with 8 to 15grams of epoxy-resin provides a suitable mixture for thepressure-accommodating mass.

The pressure-accommodating mass is poured into the bore of thepiezoelectric element 14 and is permitted to catalyze therein underambient temperatures. Subsequently, the exterior of the piezoelectricelement 14 and the associated pressure-accommodating mass are potted ina suitable potting material, such as polyurethane, epoxyresin, or thelike, in accordance with the procedures well known to those skilled inthe art. If desired, a preformed boot can be substituted for the pottingmaterials previously mentioned.

When conventional pressure-accommodating masses are utilized inconjunction with piezoelectric transducing elements at great depths, theneoprene or polyurethane sponges constituting such masses eventuallybecome compressed to such an extent as to prevent the response of saidelements to incident acoustic energy throughout their frequency range.

When microballoons fabricated from organic plastics, such aspolyurethane, or the like, are utilized in conjunction With conventionalneoprene sponges or polyurethane sponges, they will not permit thedefiective functioning of the piezoelectric elements below a depth of10,000 feet.

However, I have found that the combination of nonorganic microballoonswith the epoxy-resin body to form a pressure-accommodating mass serveseffectively as a pressure-accommodating or pressure-release medium whichwill withstand pressures up to 20,000 psi.

The relevant frequency response and electrical resistance of theelectroacoustical transducer incorporating the teachings of theinvention are essentially the same as where a piezoelectric transducingelement is constructed using foam or closed cell neoprene as a pressurerelease material. Furthermore, the air transmission of an underwaterphone incorporating the transducer of this invention appears identicalwith the standard elements conventionally utilized.

It is my belief that the explanation of the effective functioning of theelectroacoustic transducer of the present invention, as discussed andshown in FIGS. 1 and 2, can be attributed to the circumferentialexcitation of the piezoelectric element by the incident sound field.Stated in another way, the mean diameter and the mean circumference ofthe ceramic element contract and expand in accordance with the forcescharacterizing the sound field. The pressure-accommodating mass 18attenuates or absorbs the sound in the bore of the transducing element14 while at the same time being sufficiently compressible to allow theincident sound field on the external surface to cause circumferentialexcitation. If the condition exists where the sound field is incidentupon both internal and external surfaces, zero circumferential straindevelops and no transducing action is observed for the circumferentialmode.

Of course, this aspect of the performance of the transducing element inconjunction with the pressure-accommodating mass becomes even morecritical when the housing 12 in which the pressure-transducing element14 is encompassed is subjected to the extremely high pressurescharacteristic of the deep undersea depths in which such transducers maybe utilized. Adequate pressure release is required at all pressures and,at extreme depths where pressures are high, the expoxy-microballoonmaterial is vastly superior to currently used materials for the reasonsof low compressibility, the maintenance of sound-absorbing properties,good structural properties, and ease of fabrication.

I thus provide by my invention an electroacoustic transducer whoseperformance at great depths is facilitated by the incorporation, inconjunction with a piezoelectric transducing element, of apressure-accommodating mass characterized by the utilization of anorganic synthetic plastic body in conjunction with a plurality ofmicroballoons formed from such materials as glass and ceramic.

It has also been discovered that a mass of nonreactant lubricant, suchas silicone oil, transformer oil, or castor oil, which is basicallynoncompressible can be substituted for the pressure-accommodating mass18 and the microballoons 22 immersed in said oil in proportionssubstantially equal to those mentioned hereinabove in discussing the useof an organic plastic pressure-accommodating mass.

I claim:

1. In an electroacoustic transducer adapted for underwater applications,the combination of: a piezoelectric transducing element; means includinga pressure-accommodating mass of acoustic energy absorbing materialdisposed in operative relationship with said transducing element forsupporting said element against destructive deformation from thepressure of surrounding water while permitting the vibratory functioningof said element, said mass consisting of a plurality of inorganicmicroballoons supported in closely juxtaposed operative relationshipwith each other by a suspensory body with the assembly thereof disposedin operative supporting relationship with said piezoelectric transducingelement; and a housing encompassing said transducing element and saidpressure-accommodating mass.

2. In a piezoelectric transducer adapted for subaqueous applications,the combination of: a cylindrical piezoelectric transducing elementhaving openings in its opposite extremities communicating with aninternal cavity in said element; means including apressure-accommodating mass of acoustic energy absorbing materiallocated in said cavity for supporting said element against destructivedeformation from the pressure of surrounding water while permitting thevibratory functioning of said element, said pressure-accommodating massincluding a plurality of closely juxtaposed, inorganic, hollowmicrospheroids and a supporting body of organic synthetic resinencompassing said microspheroids with the assembly thereof supportingsaid element; and a housing disposed in encompassing relationship withsaid element and said mass.

3. In a piezoelectric transducer for use in underwater applications, thecombination of: a cylindrical piezoelectric transducing elementincorporating an axial bore; means including a pressure-accommodatingmass of acoustic energy absorbing material disposed in said bore forsupporting said element against destructive deformation from thepressure of surrounding water while permitting the vibratory functioningof said element, said mass including a plurality of closely juxtaposed,inorganic, hollow microspheroids, said microspheroids being supported inan organic lattice with the assembly thereof supporting said element;and a housing encompassing said element and said mass.

4. In an underwater transducer, the combination of: an elongatedcylindrical transducing element having an axially located bore thereinextending therethrough', means including a pressure-accommodating massof acoustic energy absorbing material mounted in said bore in contactwith the wall thereof for supporting said element against destructivedeformation from the pressure of surrounding water while permitting thevibratory functioning of said element, said pressure-accommodating massincluding a plurality of closely juxtaposed microspheroids which arehollow and fabricated from glass and a lattice formed from organicplastic material encompassing said microspheroids with the assemblythereof supporting said element; and an enclosure encompassing saidtransducing element and said mass.

5. In a piezoelectric transducer adapted for underwater use at highpressures, the combination of: a piezoelectric element; means includinga pressure-accommodating mass of acoustic energy absorbing materialdisposed in operative relationship with said element for supporting saidelement against destructive deformation from the pressure of surroundingwater and for preventing the pressure to which said element is subjectedfrom being efifective to render said element inoperative, said massincluding a latticed body of organic synthetic resin encompassing aplurality of closely juxtaposed, inorganic, hollow microspheroids, saidmass permitting vibratory functioning of said element during saidsupport; and an enclosure potted about said element and said mass toisolate said element and said mass from aqueous infiltration.

6. In an underwater transducer, the combination of: a piezoelectrictransducing element; means including a pressure-accommodating mass ofacoustic energy absorbing material mounted in operative relationshipwith said element for supporting said element against destructivedeformation from the pressure of surrounding water while permitting thevibratory functioning of said element, said mass including a supportinglattice of organic synthetic plastic and a plurality of ceramic, hollowspheroids distributed closely juxtaposed throughout and supported bysaid lattice with the assembly thereof supporting said element; and anenclosure encompassing said pressure-accommodating mass and element.

References Cited UNITED STATES PATENTS book (3rd ed.) Reinhold Pub.Corp., N.Y., 1960, TP 986. A2559 (pp. 150151 relied on).

Resnick, I: Performance of Glass Spheres/Epoxy Syntactic Foam in ModernPlastics, September 1965, vol. 43, Number 1, TP 986 AIM 6 (pp. 144, 146,149, 151 and 231 relied on).

BENJAMIN A. BORCHELT, Primary Examiner. P. A. SHANLEY, R. M. SKOLNIK,Assistant Examiners.

1. IN AN ELECTROACOUSTIC TRANSDUCER ADAPTED FOR UNDERWATER APPLICATIONS,THE COMBINATION OF: A PIEZOELECTRIC TRANSDUCING ELEMENT; MEANS INCLUDINGA PRESSURE-ACCOMMODATING MASS OF ACOUSTIC ENERGY ABSORBING MATERIALDISPOSED IN OPERATIVE RELATIONSHIP WITH SAID TRANSDUCING ELEMENT FORSUPPORTING SAID ELEMENT AGAINST DESTRUCTIVE DEFORMATION FROM THEPRESSURE OF SURROUNDING WATER WHILE PERMITTING THE VIBRATORY FUNCTIONINGOF SAID ELEMENT, SAID MASS CONSISTING OF A PLURALITY OF INORGANICMICROBALLOONS SUPPORTED IN CLOSELY JUXTAPOSED OPPERATIVE RELATIONSHIPWITH EACH OTHER BY A SUSPENSORY BODY WITH THE ASSEMBLY THEREOF DISPOSEDIN OPERATIVE SUPPORTING RELATIONSHIP WITH SAID PIEZELECTRIC TRANSDUCINGELEMENT; AND A HOUSING ENCOMPASSING SAID TRANSDUCING ELEMENT AND SAIDPRESSURE-ACCOMMODATING MASS.